iio: imu: inv_mpu6050: fix error path not turning chip back off

Some functions are turning the chip on and not back off in error
path. With set_power function using a reference counter that
would keep the chip on forever.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>